1. Field of the Invention
This invention relates to a method for generating halftone images from monochrome original images and, more particularly, to a method for reproducing a monochrome original image using at least two different colorants to create an image having a preselected hue and chroma while substantially maintaining the same lightness as in the monochrome original image.
2. Description of the Prior Art
In monochrome half-tone image reproduction, various shades of gray appearing in a continuous tone original image are reproduced on paper by a single tone ink. To simulate the multiple shades of gray in the original, the continuous tone image is converted into a half-tone image. Half-tone images comprise a plurality of different size dots of a uniform optical density level. The human eye integrates the dots with the background and is fooled into believing that it sees a multiplicity of continuous tone gray shades, the apparent gray level being a function of the size of the dots per unit area.
Recently, computer implemented methods have been developed, which apply algorithms to digital image data representing a continuous tone image to generate information which correlates the original image optical density values to percent dot area coverage in a halftone system needed to reproduce the original density range, preferably while taking into account the limitations in the optical density reproduction range inherent in a particular reproduction system. There are a number of algorithms available for generating half-tones from continuous tone data; U.S. Pat. No. 4,654,721 issued to Goertzel et al. and U.S. Pat. No. 4,667,250 issued to Murai, provide good examples of this technology. The result of these methods may be viewed as a transfer function which is applied to convert input data representing an original continuous tone image optical density to output data representing a percent dot area in the half tone reproduction.
In most applications the generated data is used to drive a recorder which exposes a photosensitive film creating dots sized in accordance with the data. The film after development is used to expose a lithographic plate which is then used on a press to apply a colorant, i.e., a colored ink, usually black in cases of monochromatic reproduction, to reproduce the original image.
The reproduction of full color images, which attempt to faithfully reproduce the color gamut of an original colored image using halftone technology is also well known in the art. Such technology involves a plurality, usually four, of overlapping images printed in juxtaposition, each with a different colorant. Each of those images is a halftone representation of the original representing the original as viewed through a color separation filter. Specific filters and colorants have been developed, and are well known in the art. Typically these filters are red, green and blue, while the colorants used are cyan, magenta, yellow and black. Maintaining color fidelity (i.e., hue, chroma and lightness using the terminology typically used to describe color perception space) from original to reproduction has been the subject of numerous inventions and scientific disclosures. See in general by Michael H. Bruno, published by GAMA Communications, Salem, N.H., 1986 edition, for a good discussion of some of the problems and solutions to faithful color reproduction in the printing industry.
In addition to images reproducing or at least attempting to reproduce the full color gamut of an original multicolored image in halftone, there is another form of color reproduction, in which more than one colorant is used in the reproduction of an image, but where the original image is a monochromatic image, and no attempt is made to generate an image having the appearance of a multicolored image as would be created using the color separation process briefly described above, i.e., an image with a multiplicity of hues and chroma. In this process, the reproduced image has a single hue and chroma but the lightness has been preserved substantially the same as that of the original image.
This type of image reproduction is known in the printing trade as "duotone", "tritone", "quadratone", etc., depending on the number of colorants used. A problem encountered when attempting to make "duotones", "tritones", etc., is that the percent dot areas covered by the colorants cannot be simply added numerically to duplicate the original monochromatic image while maintaining hue, chroma, and lightness. Assume for example that a desirable transfer function has been established which acceptably reproduces in half tone, an original black and white image density range. This density range is the lightness of this image. For each lightness value there is a corresponding percent dot area that will be printed, based on this transfer function. Further assume that we desire to create from the same original monochromatic, black and white image an image having a selected hue and chroma resulting from a combination of two colorants, e.g., a cyan and a black ink, so that the reproduced image has a bluish tint and that we want the bluish tint to be about 10. This means that the reproduction will be created using a combination of two images, a cyan and a black printed one on top of the other in superposition. If we want to maintain the same lightness overall as in the original, each lightness value in the original must be divided into two components one for the cyan image and one for the black. However, if we want to introduce 10% blue in the image, we cannot simply calculate for each original lightness value, a cyan ink dot area equal to 10% of the dot area called for by the monochrome transfer function, and for the black ink 90% of that same dot area. The results after superposition do not reproduce an image with the same lightness as the original.
At present, particularly in the Graphic Arts, there is a need for predictably generating duotones, tritones, quadtones, etc., from monochromatic originals. More particularly, there is need for a method whereby given an original monochrome image, and a particular transfer function selected to reproduce this image in halftone with tone values, that is, lightness, which are desirable, one may readily derive new transfer functions for preselected colorants such that images generated for each colorant using the derived new transfer functions for each colorant will produce an image when combined in superposition as by printing, which image will have a preselected hue and chroma, and in which the tonal range, i.e., lightness, will be the same or substantially the same as in an image produced using the transfer function selected for the original monochromatic reproduction.